Hi… is it possible to make some kind of job which periodically deletes old items from trash (e.g. items which have been deleted for more than 15 days)
Yes. I think there is a php script that can be run from the command line in the scripts directory of your Matrix installation. You would setup a cron job to run this periodically.
But it is not going to leave assets that have been in the trash for less than 15 days. There is no script to do this currently.